How are your device RTT average numbers in your Z-Wave table, for those devices where you are experiencing intermittent delays? Any in particular stand out w/larger (worse) values?
RTT Numbers are all over the board, I'm seeing from 48ms up to 410ms. No visible rhyme or reason why some are direct connected to the hub (with 300+ms) others are through repeaters with lower (100+ms).
The Z-Wave routing process is supposed to find the best routing for each device, but I think we've always felt that to human eyes it sometimes looks quite odd. Luckily in many cases odd routing doesn't result in performance issues - e.g., I've had devices routing through four others (including wacky cross-house jumps) and still work w/out issues. 
It can take time for devices/routing to settle, sometimes more days than expected to work it out, but there are obviously cases where no matter how long you wait the end result can still be sub-par.
There have been a small number of issues w/the antennas. Have you tried swapping them to see if that makes any difference?
I had not switched the antennas, I had removed and verified they were properly installed. I'll switch them and give it overnight to see what happens.
Low likelihood, but easy to check and rule out. 
See here for more info on POE splitter issues: [2.3.5.113] C8 Z-Wave dropout at random times of the day
@brad5 and I are a couple of the people cursed from POE splitters with the C8. C7 was rock solid, and even using the same POE splitter with the C8 (MicroUSB to USBC adapter), I have issues. I have tried 3 different POE splitters, as well as source power from a Unifi POE switch and a 30W POE injector and nothing seems to work like the AC brick does. It's very weird, if I use the provided power brick, everything is perfectly fine. If I switch to POE power, things fail. Though, others have been using POE with C8 and their setups have been totally fine. I believe @brad5 is still seeing issues, even with the provided power brick, so I don't believe everything is POE related.
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Thanks for the info. I have switched to the brick and we shall see what happens. I also moved it back to the original C7 location as well. The power specs on the brick must be special because I grabbed one of my iPhone bricks without thinking and the C8 would not even show power. Why o Why do they have to be special.
Not sure why the other brick didn't work. Nothing special with this one, its 5V 1A. Any 5V brick should power it up without issues
Well, that did it for me as well. Swapped the POE splitter (after reading this) for the original power supply, and BAM every single device is working and my RTTs are all less than 10ms. My eyes aren't what they used to be but it looked like the power supply had output 5.7V listed on it, instead of 5.0. That would make sense, the hub would function, but potentially not have enough power for the radio(s).
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Has anybody ever tried a POE+ or POE++ switch to deliver the power vs a straight POE port?
Examples of the Stats of offending devices (i.e. devices often changing route, which I believe is the cause for the delays):
Device 1: (Aeon multi sensor 6: 8 ft from the hub)
PER: 0, RTT Avg: 317ms, LWR RSSI: 29dB Neighbors: 22, Route Changes: 12. / 01 -> 0B 100kbps
Device 2: (Aeon heavy-duty switch: 5ft from the hub)
PER: 2, RTT Avg: 5ms, LWR RSSI: 42dB Neighbors: 31, Route Changes: 6 / 01 -> 49 100kbps
18 hours ago, I restarted the hub and immediately afterward ran a repair for these specific devices. Then I restarted the hub again.
You'll notice that these devices changed routes many times since the last restart 18 hours earlier. Apart from one or two other devices which have changed route once since the restart, none of the other 60 devices has changed route since the restart.
Can you post your z-wave details page in it's entirety?
Overall your mesh looks solid but those 2 items may overall be problematic. First create virtual switches/sensor and label them temp-switch and temp-sensor. Then use Device Swap and run those virtual switches/sensors against the swap. Then do an exclude (don't just forcibly remove them). Make sure they are gone from both device list and z-wave details page. Once that's done watch your mesh. Is it good? If so, add the devices back. Does behavior return? If so you have a couple of bad apples spoiling the bunch.
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My C7 was plugged into a switch with POE+ auto detecting, it never fired up on just the ethernet so the addition of the splitter. I can't imagine that has changed with C8.
Will report back. I am, however, convinced the issue is related to the distance from the hub. It is too much of a coincidence that the devices with problems are the three devices closest to the hub. Why would these devices sometimes route via three of four other devices at the outskirts of the house? But that is just a gut feeling - will report back.
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Well after removing the POE adapter and going with the provided wall wart. My directs went from 23 to 30 and the only RT Avgs above 10 are the farthest away from the hub. The only above 10 is at 12 and its literally the closest one to the hub at 3 feet. Everything behaved last night. I will keep an eye on it.
My personal thought is if you can't use competing USBC chargers(and POE) this interface should NEVER have been a USBC style input.
I would like to hear from the big guys on their comments about this!
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I'm using a cheap Amazon POE splitter w/out issue on my C8...actually a micro-usb splitter w/a micro USB > USB C adapter between the POE and the hub, so even more janky than most configurations would be.
I don't think this is a all or nothing "POE doesn't work" problem, and I thought from what we were seeing it might be linked (in part at least) to those using Unifi POE.
That's why we need clear specs!
Recently I realized that my Unifi LR 6 AP requires a POE+ connection because the POE connection is inadequate. Unfortunately POE+ switches are a little too pricey at the moment so I went with an adapter.
This can save a lot of headaches of people rebuilding their Zwave networks if the problem is more simply related to power issues.
HE doesn't officially support POE/POE splitters and likely hasn't done any testing.
If changing power supply on any device I would match up to the specs of the in-box power supply. If there are some anomalies w/some POE splitters/injectors that don't play well w/the hub for some reason, I don't think HE would be able to cover all those potential problems, nor would they (or we!) have expected any to arise, given previous hubs have been OK w/a range of injectors/splitters. This POE issue is new and I would say quite surprising.
